Problem: $\dfrac{1}{10} + \dfrac{11}{100} = {?}$
Answer: The fractions must have the same denominator before you can add them. Rewrite $\dfrac{1}{10}$ as $\dfrac{10}{100}$ $\dfrac{10}{100} + \dfrac{11}{100} = {?}$ $ = \dfrac{21}{100}$